1. Field of the Invention
The present invention relates to a multi-layered anti-reflection coating to be applied to a substrate, and more particularly to an anti-reflection coating for use with a glass substrate to reduce the reflectance of light.
2. Description of the Prior Art
A considerable number of multi-layered anti-reflection coatings have been suggested in the prior art for a primary design purpose of ensuring that residual reflectance will be held to a relatively small value over the entire range of the visual spectrum. For example, multi-layered anti-reflection coatings having six or seven layers are suggested in Japanese Laid-Open patent application Nos. 56-55902, 52-76943, and 54-58037, and U.S. Pat. No. 3,960,441.
On the other hand, various types of automatic focus detecting device for cameras have been proposed recently in which a focusing condition of an objective lens is detected automatically. Here, a so-called active type automatic focusing condition detecting device is suitable for correctly detecting the focusing condition of the objective lens when the camera is directed to a dark object. The active type automatic focusing condition detecting device is constructed so that illuminating light is projected from the device side to a target object to be focused and the focusing condition is detected by receiving the light reflected by the target object.
Here, if such an active type automatic focusing condition detecting devices is mounted on a single lens reflex camera, it is desirable that the illuminating light be projected through the objective lens and that the light reflected on the target object be received by the light receiving means through the objective lens, in order to decrease the parallax between the objective lens and the optical system of the active type automatic focusing condition detecting device. Furthermore, a wavelength of the illuminating light should be selected within the infrared light range or near infrared light range, in order to avoid the blinking of the target person caused by stimulation by the illuminating light of the eyes of the target person. If the blinking of the target person can not be avoided, an undesirable photograph in which eyes of the target person are closed is taken.
However, the proposed conventional designs of multi-layered anti-reflection coatings as described above have sufficient anti-reflection effect with respect to the incident light having wavelengths within visible light range. Therefore, if the above active type automatic focusing condition detecting device is mounted on a camera when using an objective lens in which such conventional multi-layered anti-reflection coating is provided, most of illuminating light emitted behind the objective lens is reflected on one of the lens surfaces in the objective lens without illuminating the target object, or most of the light reflected from the target object is reflected on one of the lens surfaces in the objective lens without being incident on the light receiving means of the automatic focusing condition detecting device. Thus, it is difficult to correctly detect the focusing condition of the objective lens with respect to a target object positioned relatively far from the camera.